Technology update

Oct 25, 2004

Silicon nanocrystals made easy

Engineers at the University of Minnesota in the US have developed a new technique for making silicon nanoparticles in plasmas at room temperature. The method overcomes problems encountered with existing plasma-based approaches and can produce crystalline nanoparticles with a uniform size. The team says that the crystals could be used to make novel electronic devices, such as single-nanoparticle transistors (A Bapat et al. 2004 arxiv.org/abs/physics0410038).

Crystalline silicon has better characteristics than its amorphous counterpart for applications in high-speed electronics, but existing plasma synthesis techniques almost always produce the amorphous variety. Moreover, the nanoparticles produced with these methods either contain large numbers of defects or vary in size by a large amount.

The new technique developed by Uwe Kortshagen and colleagues has none of these disadvantages and produces virtually defect-free crystalline nanoparticles with a narrow range of sizes.

Kortshagen and co-workers begin by injecting a dilute mixture of 5% silane (SiH4) in 95% helium and argon into a narrow quartz tube about 23 centimetres long. They then apply around 200 watts of power at a frequency of 13.56 megahertz to a ring-like electrode that is about 10 centimetres away from the ground electrode. The resulting plasma is unstable and is made up of a filament of bright plasma globules. Existing approaches to plasma synthesis use stable, uniform plasmas.

The high-energy electrons in the plasma decompose the silane gas into its constituents and the silicon atoms released in this way then recombine to form silicon particles. Transmission electron microscopy reveals that the nanoparticles are all between 20 and 80 nanometres in size and are predominantly cubic-shaped.

"At present, we do not completely understand what causes the well-defined shape of our particles, or why they form crystals," Kortshagen told PhysicsWeb. "However, we believe that the filamented plasma plays an important role in that it heats the particles to temperatures that are several hundreds of degrees hotter than the surrounding gas. The atoms in a particle can then readjust themselves, which allows it to find an energetically favourable shape."

The team now hopes to extend its process to other commercially important materials, such as gallium arsenide and gallium nitride.